There is a need for multilayer heat-shrinkable films and articles of manufacture made therefrom, which have high impact strength, especially at elevated temperatures, high free shrink at 185° F., high modulus, high gloss and package presentation, good sealability and seal strength, and stack/overlap sealing capability, and which can be easily oriented. This combination of features is not currently available. There is also a strong desire in the marketplace for thin films which also possess the above-described combination of features. The process of downgauging films without sacrificing performance attributes not only utilizes less polymeric material (which is better for the environment) but also lowers the cost to the end-user.
Recently it has been discovered that certain commercially-available bags can be sealed when stacked on top of one another, i.e., without sticking to one another. This non-sticking characteristic provides an advantage for packaging in a vacuum chamber, because the chamber, although typically having only one sealing means, has more than enough space therewithin for multiple bagged products which are to be sealed after evacuation of the atmosphere from the chamber. Thus, the non-sticking feature enables the evacuation and sealing of more than one bag at a time in a vacuum chamber, thereby increasing the production rate of the vacuum chamber packaging apparatus.
U.S. Pat. No. 5,336,549, to Nishimoto et al., discloses a heat-shrinkable film which can be made into bags. Apparently, users of this film, which is commercially available, hive discovered that bags made from the film can be stacked on top of one another during sealing, without sticking to one another (i.e., the bags are “stack-sealable”). This enables the output of vacuum chamber packaging machinery to be, for example, doubled, if two bags are stacked on top of one another and simultaneously sealed.
The film disclosed in the '549 patent has an outer layer of a polyester, and an intermediate layer of a polyamide having a melting point of higher than 160° C. and lower than 210° C. Although Nishimoto et al discloses a large group of polyamides for use in an inner layer, together with various polyesters for use in an outer layer, Nishimoto does not disclose the use of an inner layer comprising polyester.